Which type of muscle is made of small, delicate muscle cells and is found throughout the internal organs of the body?

Smooth muscle is characterized by its small, delicate muscle cells and is predominantly found in the walls of internal organs. This type of muscle is involuntary, meaning it operates without conscious control, facilitating vital functions such as digestion and the regulation of blood vessel diameter. The arrangement of smooth muscle cells allows for slow, sustained contractions, which are essential for the functioning of organs like the intestines, bladder, and blood vessels.

In contrast, myocardial muscle, or cardiac muscle, is specialized for the heart and is not found throughout the internal organs. Skeletal muscle is striated and primarily responsible for voluntary movements, while fibrous tissue is not a type of muscle but rather connective tissue that provides support and structure to various parts of the body. Thus, D is the correct choice as it accurately describes the characteristics and location of smooth muscle.